At present, the art in common use for obtaining such semifinished products requires that the continuous plate coming from the extrusion phase should be cooled and subsequently passed under punching machines which cut the material in order to create holes at previously stated positions. However this procedure involves some drawbacks which can affect both the cost and the quality of the product. In fact, a cold punching gives inevitably rise to the formation of an important quantity of scraps, above all when the holes have a bigger surface than the solid portions delimiting them, as in the case of trellis and the like. Furthermore, it is to be observed that the punching operation causes, at some predetermined points, an interruption or cutting of the fibers forming the structure of the plastic material. Obviously, the cutting of these fibers causes the structure of the finished product to be weakened, so that the latter can only be submitted, in use, to reduced tensile stresses, while it is desirable that the bearable stresses should be high both during the assembling and the use stages.